E.H. CROOK,G. F. TRBBBI, AND D. B. FORDYCE
3592
ditions of maximum dehydration of the ethylene oxide chain on electrolyte addition are prone to maximum hydration of the ethylene oxide chain on urea addition. Acknowledgment. The author wishes to express
his gratitude to Lever Brothers Co. for permission to publish this paper, to X r . D. Manning and Mr. E. Beyer for carrying out the experimental work, and to Mr. W. Pease for synthesizing the ionic detergents.
Thermodynamic Properties of Solutions of Homogeneous p,t-Octylphenoxyethoxyethanols(OPE,- lo)
by E. H. Crook, G. F. Trebbi, and D. B. Fordyce Research Laboratories, Rohm and Haas Co., Philadelphia, Pennsylvania
(Received A p r i l 8 , 2.964)
Surface tension as a function of concentration and temperature has been determined for aqueous solutions of single species p,t-octylphenoxyethoxyethanols(OPE1From these data surface tension a t the critical micelle concentration (yc.m.c.), area per molecule, and c.m.c. have been obtained and are presented as a function of temperature and ethylene oxide chain length. y o decreases with temperature, the dependence being greatest for compounds with ethylene oxide chain lengths greater than five EO units. This behavior may be attributed to the dehydration of the surfactant molecules which form a lower surface energy film at the air-water interface. Area per molecule at the air-water interface increases as a function of temperature due to the greater surface area swept out by the molecules with increased thermal-kinetic motion. Applying the pseudo-phase model of micellization, changes in enthalpy (AHmT)and entropy (ALLT) of micellization are calculated for OPEl-lo. AHmTand ASmT increase with increasing EO chain length and decrease with temperature. With increasing EO chain length, this corresponds to a greater positive energetic contribution to the micellization process due to the breaking of an increasing number of hydrogen bonds, and with increasing temperature at constant EO chain length, there is a reduced contribution to the energetics of the micellization process because of a lesser degree of initial hydrogen bonding.
Introduction In a previous investigation,l the surface tension of single species OPE1-loas a function of concentration was determined a t 25’. At this temperature, several conclusions were drawn as to the effect of ethylene oxide chain length on the surface physicochemical properties of these compounds. Only a few recent investigations2-10 have dealt with the effect of temperature on the surface tension properties of nonionic surfactants. It is commonly known that temperature has a T h e Journal of Physical Chemistry
drastic effect upon the bulk properties of certain nonionic compounds. This is demonstrated by the SOcalled “cloud point” phenomenon.11112 In this investigation a systematic study of the effect of temperature (1) E. H. Crook, D. B. Fordyce, and G. F. Trebbi, J . P h y s . Chern., lg8’ (1963). (2) M.J. Schick, ibid., 67, 1796 (1983). (3) M. J. Schick, J . Colloid Sci., 17, 801 (1962). (4) J. M. Corkill, J. F. Goodman, and R. H. Ottewill, Trans. Faraday sot., 57, 1627 (1961).
673
THERMODYNAMIC PROPERTIES
3593
OF p,~-~OCTYLPHENOXYETHOXYETHANOLS
upon the surface tension properties of solutions of homogeneous OPE1-lo is presented.
Experimental Materials. Details of the preparation and characterization of single species OPE1- lo have been presented el~ewhere.'.'~All of the water used in the preparation of solutions was distilled from alkaline permanganate solutioin and had a specific conductance of
n
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M O L E S OF E T H Y L E N E
7
8
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OXIDE
Figure 2. Plot of Y ~ . ~us.. ethylene ~ . oxide chain length ( t = 15-85'),
5 ) , and critical micelle concentration, c.ni.c. (Fig. 6 and 7), were determined. Calculated values of the enthalpy (AHmT)and entropy (ALLT) of micellization which were determined from plots of log c.ni.c. us. 1/T data are presented in Fig. 8 and 9. A(AH,) us. t'C. is presented in Fig. 10. yo.m US. Temperature. From Fig. 2 it is clear that OPE3-, are the most surface-active members of the OPE series a t all temperatures that were studied. It is noteworthy that the relative order of surface activity within the series varies with temperature. This is to be expected since the HLB (hydrophile-lipophile balance) of the individual molecules will change with temperature. For example, at 15' OPE, is the most surface active molecule of the series, while between 35 and 65' OPE, becomes the most surface-active entity, and finally OPE5 and OPE6 are the most surface-active molecules a t higher (75-85') temperatures. This effect occurs as the molecules become more hydrophobic as a consequence of increasing dehydration of the EO chains as the temperature increases. Also of interest is an apparent limiting surface tension (25.8 dynes cm.-l) which occurs in the OPE series in the range of 75 to 85'. This is necessarily related to a unique value of HLB The Journal of Physical Chemistry
T E M P ER A T U R E
'C
Figure 3. Plot of Y ~ . ~us.. ~temperature . (OPEl-lo).
which gives rise to an optimum molecular packing at the air-water interface of OPE-water systems. With shorter EO chain length members, values of yc.m.a. approach the surface tensions of alkylated phenols ( y = 39-41 dynes cm.-l) as the EO chain length decreases. It is likely that with octyl phenol and perhaps OPE1 an insoluble monolayer forms at the airwater interface. This could account for the observed high surface tension values. Maintaining the EO chain length constant (Fig. 3), the surface tension at the c.m.c. decreases with increasing temperature, which behavior is analogous to the surface tension dependence of pure liquids, e.g., benzene, hexane, or molten KaC1. The decrease of surface tension a t the c m c . with temperature, however, is nonlinear while conventional liquids usually show a linear decrease of surface tension with teniperature. Such deviations from linearity for all of the systems studied can be related to the effect of teniperature on the interactions between the surfactant niolecules and the water molecules. Such interactions are present and sensitive to temperature because of the large degree of hydrogen bonding which exists in the system and, in fact, is responsible for the solubility of the OPE molecules. With longer chain length OPE'S (e.g.,
THERMODYNAMIC PROPERTIES
---
I
1
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I
3595
OF p,t-OCTYLPHENOXYET'HOXYETHANOLS
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MOLES
OF E T H Y L E N E O X I D E
Figure 4. Area per molecule us. ethylene oxide chain length ( t = 25, 55, and 85").
OPElo) the EO chain is greatly hydrated. As thermal energy is imparted to the system, many of the existing hydrogen bonds are ruptured and a corresponding decrease in the surface tension occurs as the hydrophobicity of the molecules increases. For shorter chain length OPE'S (e.g., OPE4)the dehydration effect will be somewhat less pronounced since the surfactant molecule binds considerably less water, i.e., is more hydrophobic, to begin with. I n fact, a t higher temperatures, the surface tension a t the c.ii1.c. of OPEI-C,either approaches a limiting value or in some cases, actually increases. This behavior is caused by the continuously changing HLB of the surfactant as the temperature increases and with the shorter EO chain lengths can lead to a less surface-active structure. I n interpretation of the properties of EO containing molecules dissollved in aqueous systems, two opposing thermally controlled effects n u s t be considered : (i) the dehydration of the EO chains which results in an increase of hydrophobic character of the molecule with increasing temperature, and (ii) increase of solubility with increasing temperature due to kinetic-thermal effects. With longer EO chain length OPE'S, the first effect overbalances the theriiial solubility effect, but
20
30
40
, 50
70
60
T E M P E R A T U R E 'C
80
e OPE, A OPE? OPE1 POPP E !l
90
.
Figure 5 . Area per molecule us. temperatu'le (OPELAO).
with shorter EO chain length inolecules, which have only a few oxygen atoms available for hydrogen bonding, the second effect plays an important role and in some instances (e.g., OPEI--6) predominates. This trend will be further demonstrated in the discussion of c.ii1.c. as a function of temperature. A r e a per Molecule us. Temperature. From Fig. 4 and 5 it is clear that the area per molecule a t the air-water interface is an increasing function of EO chain length and temperature. The result obtained as a function of EO chain length has been reported p r e v i o u ~ l y as ~,~~ being due to poorer packing a t the air-water interface since the hydrated, coiled EO chains sweep out a greater surface area as their length increases. As the EO chain length approaches zero, the area per molecule approaches that of the hydrophobe the p,toctylphenyl group. The increase of area per molecule a t the air-water interface with temperature is presumed to be primarily EL kinetic-thermal effect. An increase of niolecular nio(14) M . J. Schick, S. hl. Atlas, and F. R. Eirich, J . Phys. Chem., 66, 1326 (1962).
(15) M. J. Schick and E. A. Beyer, J . Am. Oil Chemists' Hoc., 40, 66 (1963).
Volume 68, Number 12
December: lQSL$
3596
E. H. CROOK,G. F. TREBBI, AND D. B. FORDYCE
I
0
+
I
1
3
4
MOLES
5
6
7
8
9
10
OF E T H Y L E N E O X I D E
Figure 6. C.m.c. us. ethylene oxide chain length ( t = 25, 55, and 85").
tion with increasing temperature results in poorer packing of the adsorbed molecules and a consequent increase of area occupied per molecular unit. This thermal effect is opposed by the influence of the decreased hydration of the EO chains (which consequently sweep out less area under static conditions) which undergo further dehydration with increasing temperature. With all of the OPE'S studied, the former area-increasing effect predominates over the latter area-decreasing effect. C.m.c. us. Tempemture. The data plotted in Fig. 6 show that the c.m.c. a t constant temperature is an increasing function of EO chain length within the OPE series. This result is in agreement with previous studies1t15-1* with nonionic surfactants. Such behavior is directly related to the increase of hydrophilicity of the OPE molecules with increased EO chain length; i.e., longer EO chain length surfactant molecules being more water soluble because of increased oxygen atom content, form micelles at higher concentrations than do the shorter EO chain length molecules. In Fig. 7 for the surfactants, oPE5-1~the c m c . initially decreases, then increases, as the temperature of the system is increased. The initial decrease of the c.m.c. with temperature is undoubtedly a result of the decreased solubility of the surfactant because of the T h e Journal of Physical Chemistry
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45
TEMPERATURE
I 55
I 65
OPE9 OPE10 I 75
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Figure 7. C.m.c. us. temperature (OPE,-,,).
rupture of hydrogen bonds, i.e., the onset of micellization occurs a t a lower concentration. As the teniperature increases further, the thermal solubility effects begin to exert their influence and finally predominate as the c.m.c. attains a minimum value and finally increases with temperature. Structurally, this corresponds to an initial increasing aggregation of micelles followed a t higher temperatures by a partial disintegration of these highly dehydrated aggregates into smaller aggregates. Such behavior has been observedlB with some of these materials (OPEb-,). One notes first that with increasing temperature the turbidity of the aqueous system increases, reaches a maximum, and then decreases, thus reflecting a corresponding change in aggregation number. The c.m.c. values of OPEp4 increase monotonically with temperature. The insolubilizing effect of dehydration is much less pronounced and indeed the kineticthermal solubility effects predominate. The average (16) L. Hsiao, H. N. Dunning, and P. B. Lorena, J . Phys. Chem., 6 0 , 657 (1956). (17) F. V. Nevolin, T. G. Tipisova, N. A . Polyakova, and A. M. Semenova, J . prakt. Chem., 15, 206 (1962). (18) H. Lange, Proc. Intern. Congr. Surface Actiuitu, 3rd Cologne, 1, 279 (1961). (19) E. H. Crook, D. B. Fordyce, and G. F. Trebbi, J . Am. Oil Chemists' Soc., 41, 231 (1964).
THERMODYNAMIC PROPERTIES OF ~,~-OCTYLPHENOXYETHOXYETHANOLS
359”
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.
.
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OF
ETHYLENE OXIDE
Figure 8. hHm us. ethylene oxide chain length ( t = 25, 45, 65, ttnd 85’).
slope of the curve is more positive the shorter the EO chain length, which fact is consistent with the decreased dehydration of the EO chain of the smallest OPE molecules. One of the referees has pointed out that some of the systems studied may have precipitated as the temperature was raised, so that kinks in the surface tension us. concentration curves may correspond to solubility effects rather than to c.m.c. effects. For solutions of all the compounds studied (OPEl-lo) insolubility is already present or attained in the 15-85’ temperature range (e.g., 1% solutions of OPE1-6 all have “cloud points” below 01’ while 1% solutions of OPE,-10 have ‘(cloud points” of 28, 54, 68 and 7 8 O , respectively) a t concentrations in excess of the c.m.c. This “cloud point” corresponds to an aggregation of micelles such that a distinct phase separation occurs in the system, as is evidenced by turbidity. As is shown in Fig. 7, no simple correlatlion exists between this cloud point phenomenon and the dependence of the c.In.c. on tern.perature, since solutions of OPE4-, which exhibit turbidity over the entire temperature range show both positive and negative slopes within the c.1n.c. us. temperature plot over the temperature interval studied. Also, the compounds, 0PE7-10, all show a reverse of
sign of slope of c.m.c. us. temperature at 4 5 O , thus being independent of EO chain length. Consequently, it is believed that the obtained c.m.c. values are indeed indicative of a prima,ry niicellization process which is coincidental in some cases with aggregation rather than due to any simple insolubility effects which are independent of a micellar structure. Thermodynantics of Micellixation. Two general theories have been proposed to account for the therniody nainics of niicellization, L e . , the phase separation In principle, if, mode120-22 and the mass action is identical to treat micelle formation either as a inas8 action equilibrium or as a p s e u d o - p h a ~ e . ~If, ~ however, the aggregation numlber of the micelle is small, the masti action model is used, while if the aggregation number is; large, the phase separation model is applied.25 Since the aggregation numbers of the micelles of ethoxylatee
(20) A. E. Alexander, Trans. Faraday SOC.,38, 54 (1942). (21) K. Shinoda, Bull. Chem. SOC.J a p a n , 26, 101 (1953). (22) E. Hutohinson, A. Inaba, and L. G. Bailey. 2. p h y s i k . Chem (Frankfurt), 5, 344 (1955). (23) J. N. Phillips, T r a n s . Faraday SOC.,51, 561 (1955). (24) K. Shinoda and E. Hutchinson, J. P h y s . Chem., 66, 577 (1962). (25) K. Shinoda, et al., “Colloidal Surfactants,” Academic Press, London, 1963, p. 37.
Volume 68, Number l a
December, 196,!+
3598
E. H. CROOK,G. F. TREBBI, AND D. B. FORDYCE
0 25' C 0 45. c 65. C
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I
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4
MOLES
I
6
'
7
I
0
I
9
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I
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I 4 5
OF E T H Y L E N E O X I D E
c.m.c./bT),
(1)
Since the micellization process is considered a phase transition, AF,' = 0, and thus =
I
1
I
I
25
35
45
55
65
75
I
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to unity and thus does not contribute to any errors in thermodynamic functions derived from c.m.c. as a function of temperature data. This certainly applies to single species OPE1-10 in which the mole fraction c.m.c. varies from 7.8 X lo-' to 6.1 X (for OPE1 and OPElo, respectively). In this investigation the aggregation number of the compounds as a function of temperature was not determined. If the aggregation number is large (as it i ~ 1 ~ , ~with ' 3 1ethoxylated ~~ adducts of alkyl phenols) the fraction of solute in the aggregated form will be negligible and the correction to be applied to the change in enthalpy and entropy will be very small and can be negle~ted.~'On these grounds it is believed that the thermodynamic parameters derived from the temperature dependence of the c.m.c. of
AH,'/T
Recent criticism2gof this method has centered about the need for values of AHmTand ASmT which are corrected for: (i) the change of activity with concentration of the surface-active molecules, and (ii) the change of aggregation number with temperature. With compounds having small c.m.c. values it has been shown30 that the change of activity with concentration is equal The Journal of Physical Chemistrg
0
I
Figure 10. Increment of AH,,, per EO unit us. temperature (average EO chain length = 1.5, 3.5, 5.5, 9.5).
of alkyl phenols are greater than 100 for EO chain lengths of ten or less,14.z6~27 the pseudo-phase model has been applied to the present results. With this model, the c.m.c. corresponds to the saturation solubility of the molecular species. If this concentration is exceeded, a new phase is formed. Stainsby and Alexanderz8have proposed the calculation of the change in enthalpy (AH,') and entropy (AS,') of micellization from the temperature dependence of the c.m.c. according to a Clausius-Clapeyron type of relationship - RTz(d In
0
I
t
Figure 9. ASm us. ethylene oxide chain length ( t = 25, 45, 65, and 85").
AHmT=
" A
0
(26) C. W. Dwiggina, Jr., R. J. Bolen, and H. N. Dunning, J. Phys. Chem., 64, 1175 (1960). (27) P. Becher, J. Colloid Sci., 16,49 (1961). (28) G. Stainsby and A. E. Alexander, Trans. Faraday SOC.,46, 587 (1950). (29) K. Shinoda, et al., ref. 25, p. 129. (30) K. Shinoda, et al., ref. 25, p. 36. (31) K. Shinoda, et al., ref. 25, p. 36.
THERMODYNAMIC PROPERTIES
O F p,t-OCTYLPHENOXYETHOXYETHANOLS
OPE1-loare good approximations to the more accurate values which could possibly be obtained via calorimetric determinations of the heat of d i l ~ t i o n . ~ ~ - * ~ Values of A H m T and ASmT as a function of EO chain length a t various temperatures are presented in Fig. 8 and 9. AHmTand AX,,T at a given temperature become less negative and in some cases positive as a function of increasing EO chain length. AHmTand ASmT for a given EO chain length molecule decrease with increasing temperature: in some cases (OPE4-lo) passing from positive to negative values while in others becoming increasingly more negative (OPEI-3). There are two main structural processesa5which oppose one another during micellization; (i) the destruction of a considerable portion of the "iceberg" water structure about the monomeric units of surfactant during their incorporation into the micelle with a consequent desolvation of water molecules, and (ii) the rearrangement of a substantial number (>loo) of randomly oriented monomeric molecules into a well-ordered micellar structure. From an entropy viewpoint the former process should lead to a positive change in entropy in the trainsition from a monomeric to a micellar state, while the latter process would lead to a negative value of the change in entropy: With OPE4-lo at 25' the change in entropy upon micellization becomes more positive as the EO chain length increases. This result is consistent with the desolvation of IIZO molecules outweighing the ordering process of the surfactant molecules, and increasingly so as the EO chain length and consequently the number of hydrogen-bound water molecules increase. At EO chain lengths less than four, the change in entropy still increases with ]EO chain length but the ordering effect of inicellization predominates over the desolvation effect simply because of the reduced number of hydrogenbound water molecules. With increased temperature, e.g., 65 or 8 5 O , the ASm' values are all negative, which indicates that the micellar ordering process is more important than the desolvation effect over the entire range of OPE1-lo. This result is entirely consistent with(the less solvated initial structure of the monomeric surffactant molecules a t these temperatures (which can no longer overbalance the ordering process that occurs during micellization) , The present results are entirely in accord with those
359!3
of SchickPwho calculated AHmT and A S m T for molecularly distilled fractions of EO adducts (ranging from 10 to 50 EO units) of t-octylphenol and nonylphenol. In the temperature interval studied (25 to 55') with compounds of EO chain length longer than those investigated in the present work, the obtained AHmTand ASmT values are positive. This agrees with the trend of the present results obtained for the longest EO chain length compound which was studied, L e . , OPElo. In Fig. 10 is plotted the increment in AH, ( A ( A H ~ ) ) per unit increase in EO chain length as a function of temperature for several average EO chain length molecules. In general, the increment in AH, decreases as a function of increasing EO chain length and approaches a limiting value of ca. 200 cal. mole-1 in the range of ethylene oxide chain length of 6 to 10 units (and by implication a t longer E O chain lengths). At shorter EO chain lengths (OPEl.-J the increment in AH, per unit of EO is strongly temperature dependent above 35'. This is due to the strong dehydration effect of teinperature on the small number of water molecules which are bound to such short EO chains; i.e., at higher temperatures (e.g., 65') the difference in the energy of micellization between OPE1 and OPE2 is larger than the saine parameter evaluated at 35'. For the longer EO chain length inolecules (OPE,+lO), the values of A(AH,) are practically temperature-independent because thle number of bound water molecules is not as drastically increased on a fractional basis in a transition froiili OPEW, whether evaluated at 35 or 6 5 O , as would be the case with t,he shorter EO chain length coinpounds. The same explanation applies to the increment of AS, &sa function of temperature evaluated at several average EO chain lengths. Acknowledgments. The authors express their gratitude to Messrs. R. C. Mansfield and J. E. Locke for the preparation and characterization of OPE1-lo and tio Drs. K. A . Booman and F. A. Blankenship for their useful comments pertaining to the manuscript. (32) P. White and G. C . Benson, J . Colloid Sci., 13, 584 (1958); Trans. Faraday Soc., 5!i, 1025 (1959); J . Phys. Chem., 64, 599 (1960). (33) E. D.Goddard, C. A . J. Hoeve, and G. C. Benson, ibid., 61, 593 (1957). (34) E. Hutohinson, K.E. Manchester, and L. Window, ibid., 58, 1124 (1954). (35) H. S. Frank and M.W. Evans, J . Chem. Phye. 13, 507 (19451.
Volume 68,Number 12 December, 196.6
